[Testing Platform] Count tests

[thomhurst]

It'd be useful to pass in a flag to return a simple count of test nodes. This could help users/build systems perform things like parallel execution batching.

We could probably derive this from --list-tests but it would involve more cumbersome console output parsing.

This could be its own flag, such as --count-tests, or a more granular option for --list-tests

e.g. --list-tests count

[Evangelink]

I do prefer the option to the --list-tests command. Relates to #3221.

[thomhurst]

Works for me 😄

[Youssef1313]

Also related to https://github.com/dotnet/sdk/issues/49754